Ion mobility spectrometers (IMS) can identify material from a sample of interest by ionising material (e.g., molecules, atoms) and measuring the time it takes the resulting ions to travel a known distance through a drift gas under a known electric field. Each ion's time of flight can be measured by a detector, and the time of flight is associated with the ion's mobility. An ion's mobility relates to its mass and geometry. Therefore, by measuring the time of flight of an ion in the detector it is possible to infer an identity for the ion. These times of flight may be displayed graphically or numerically as a plasmagram. Other types of spectrometers, such as mass spectrometers analyse ions according to their mass-charge ratio.
To improve the ability of a spectrometer to identify ions in a sample of interest, it is suggested to modify some of the ions using a radio frequency, RF, electric field (e.g. by fragmenting them) to provide additional information which can be used to infer an identity for the ions. This may improve the ability to resolve differences between ions. Where measurements are performed in the presence of contaminants, or in difficult operating conditions, or where a sample comprises ions with similar geometries and masses etc. ion modification is one way to assist the ion mobility spectrometer's ability to detect and identify ions. Ions which are produced by this process of ion modification may be termed “daughter ions”, and ions from which daughter ions are produced may be termed “parent ions”.
It is desirable to increase the proportion of parent ions that are modified to provide daughter ions, and also to increase the energy efficiency of the ion modification process.
In the drawings like reference numerals are used to indicate like elements.